This invention relates to the field of infrared (IR) detectors, and particularly to thermal-type IR detectors.
IR detectors are finding increasing use in imaging systems, particularly for military applications. State of the art systems employ photodetector arrays which must be cooled and which utilize ancillary electronics for signal processing to provide useful outputs. The arrays are assembled in hybrid fashion so that each detector is physically and electrically interfaced with an input tap on the silicon chip which contains the electronics. In order to avoid the need for cooling and to simplify and to increase the reliability of arrays, there is a need to develop different types of detectors and techniques for interfacing the detectors and the electronics.
IR detectors are conveniently divided into two catagories: (1) photon detectors, and (2) thermal detectors. In photon detectors, the incident radiation produces electronic transitions which cause a change in the distribution of the free charge carriers which in turn changes the conductivity or output voltage. Photon detectors are wavelength sensitive because the electronic transitions are energy or wavelength dependent. However, this type of detector often exhibits fast response, when trapping phenomena is not dominant, because electronic transitions occur at fast rates.
Detectors in the second broad catagory of detectors, namely thermal detectors, operate by absorbing IR radiation which causes their temperature to rise. This rise in temperature causes a change in the physical, mechanical, or electrical properties of the detecting material which can then be measured to complete the IR detection function. Thermal detectors are effective at all wavelengths provided that the material is sufficiently absorbing. They are slower than photoconductive detectors because time is required to heat or cool the detecting element.
Numerous types of thermal detectors have been proposed based upon measuring particular changes in the properties of the detecting material. This invention is a thermocouple-type detector/detector array based upon the Seebeck effect. According to the Seebeck effect, a voltage is generated between the ends of two different conductors when a temperature difference is established between their common contact (the hot junction) and their ends. For application to IR detection, the common contact is exposed to the infrared radiation to cause its temperature to change as a function of the IR radiation.
For application to IR imaging, the detectors must comprise an array of minute closely spaced independent elements with a number and spacing compatible with an acceptable IR image resolution. Further, since the outputs of the elements will be sampled by a time sequenced multiplexer, the response time of the elements must be shorter than the sequence time provided.
In order to provide rapid response and to permit satisfactory resolution, thermocouple-type detectors must be very small. Silicon integrated circuit technology permits batch fabrication of very small devices and offers the advantage of integrated circuit signal processing on the detector chip. A thermopile IR detector fabricated using silicon integrated circuit technology has been described by G. R. Lahiji and Kensall D. Wise in an article titled "A Batch-Fabricated Silicon Thermopile Infrared Detector", IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol. ED-29, No. 1, January 1982, pp. 14-22. Their detector utilized bismuth-antimony or polysilicon-gold as the thermocouple materials. The "hot" junctions of the thermopile were formed by depositing these materials on a membrane forming a small window in a silicon substrate. The membrane consists of a layer of thin dielectric such as silicon dioxide on boron doped silicon. The boron doped silicon is not etched by the etchant which is used to form the openings for the windows in the silicon substrate, and it provides a support for the dielectric and for the hot junctions of the thermopiles. The boron doped silicon also increases the mass which is heated by the IR radiation and provides a path of heat loss from the hot junction to the silicon substrate. The large size of these elements, their low efficiency, and the spacing between them precludes their use for IR imaging.